Description The sandcastle worm (Phragmatopoma californica), also called the honeycomb worm or honeycomb tube worm, is a reef-forming marine polychaete worm belonging to the family Sabellarididae. It is dark brown in color with a crown of lavender tentacles and has a length of up to about 7.5 centimeters (3.0 in). The worm inhabits the Californian coast, from Sonoma County to northern Baja California.

Sandcastle worms live in colonies, building tube reefs somewhat similar to sandcastles (hence the name), which are often seen on rocky beaches at medium and low tide. The sandcastles, which have a honeycomb-like outward appearance,can cover an area of up to 2 meters (6.6 ft) on a side. They may share areas with mussel beds and are found in any place that provides some shelter, such as rock faces, overhanging ledges and concave shorelines.

The worms remain in their tubes and are almost never seen. At low tide, when above the water, they close the entrance to their tubes with a shield-like operculum made of dark setae. When submerged, they extend their tentacles out of the tube to catch food particles and sand grains. The grains are sorted, with the best ones used to keep the tube in repair, and the rest ejected. The colonies are formed by the gregarious settlement of larvae, which require contact with an existing colony to metamorphose into adult worms.

Sandcastle worms should not be confused with the similar, but more northern Sabellaria cementarium which are found from Alaska to southern California and have an amber-colored operculum.